Walmsley, 22, made his debut in last Friday’s 22-22 draw against Hull.
McCarthy-Scarsbrook said: “They have settled in amazingly, it feels like they’ve been here for years.
“Alex has just come up from the Championship but I think a year ago he was playing non-league, and just doing what he can. Then he’s done very well at Batley and he’s got scouts at Saints looking at him and now he’s here with us. The development of him is unbelievable.
“I think he’s like 6 foot 7 or something like that, he’s massive, and he’s a good lad as well. If he runs like he does in training, then I feel sorry for the fellas in front of him!”
London boy LMS is in his third season at Saints, and has barely missed a game since arriving from Harlequins. A close season trip down south enabled him to stock up on his beloved pie and mash, and he has an infectious personality that makes him centre of the dressing room, and also the best person to judge who, of the new signings, is top of the banter stakes.
He added: “Big Willie’s a quiet man but he’s very witty in the changing room, and you need that. Jordan’s another quiet one but when he comes out with line, it’s a cracker. When he’s on form, he’s on form. Alex is just this big man that sits in the corner that’s also very funny, but not as funny as me!”
The big prop, who already has two international caps, will be looking to play his way in to Steve McNamara’s World Cup plans, but says his focus is Saints.
“I say nearly all the time that all I want to do is play well for Saints, and if it does catch the eye of the selectors that are looking, and they want to pick me, then I’d love to play.